Optical pickup spherical aberration compensating method, optical pickup sherical aberration focus offset compensating method, and optical pickup device

ABSTRACT

An optical pickup ( 20 ) projects a read beam onto an optical disc ( 40 ) as an optical storage medium which is driven to rotate by a spindle motor ( 30 ) and receives its reflection. A laser beam from a laser producing element ( 21 ) passes through a liquid crystal panel ( 25 ) and guided to an objective lens ( 26 ). The panel ( 25 ) is provided to correct spherical aberration caused by an irregular thickness of a transparent substrate of the optical disc ( 40 ). A control circuit ( 50 ) changes a spherical aberration correction signal SA to carry out sampling more than once, covering a range of the output of an optical sensor ( 31 ) where the output shows large changes. The circuit ( 50 ) determines the position of a peak of an approximation curve through calculation and designates that position as the magnitude of correction. Thus, the optical pickup ( 20 ) can quickly and accurately detect the magnitude of the correction of the spherical aberration caused by an irregular thickness of the optical disc ( 40 ).

TECHNICAL FIELD

The present invention relates to a spherical aberration correctionmethod for optical pickups which are used in reading/writing compactdiscs (CD), laser discs (LD), digital versatile discs (DVD), and otherlike optical storage media, and relates also to a spherical aberrationfocus offset correction method for those optical pickups and the opticalpickups.

BACKGROUND ART

Read/write optical pickups have been used in optical readers forread-only CDs, LDs, DVDs and other similar optical discs and also inoptical reader/writer for write once or rewriteable optical discs, suchas CD-Rs, CD-RWs, DVD-Rs, DVD-RWs, DVD-RAMs, and mini discs (MDs).

Optical discs as optical storage media have a transparent substrate witha predetermined thickness to cover their recording surface forprotective purposes. An optical pickup as information readout meansreads the optical disc by the intensity of reflection when a read beamis shone on the recording surface through the transparent substrate.

However, it is difficult in manufacturing to fabricate the transparentsubstrates in all optical discs with the same specified thickness;irregularities of a few or 10 to 40 micrometers are typical. Theirregular thickness of the transparent substrate results in sphericalaberration which in turn can markedly reduce the amplitude of aninformation readout signal and/or tracking error signal. This entailsinaccurate information readout. Specifically, when changing opticaldiscs, there will likely be a difference in thickness between thetransparent substrates. The difference translates into a change inspherical aberration and without taking any measures, causes a fall ininformation readout accuracy.

The problem is addressed in Japanese published patent application2001-222838 (Tokukai 2001-222838; published on Aug. 17, 2001). Tokukai2001-222838 discloses a method involving spherical aberration correctionmeans which corrects spherical aberration in an optical system inaccordance with the magnitude of the spherical aberration, whereby thespherical aberration is corrected by observing the amplitude of atracking error signal while varying the magnitude of correction so thatthe spherical aberration correction means is supplied with an finalspherical correction assisting quantity which is the magnitude ofcorrection at which the amplitude assumes a maximum value.

The spherical aberration correction means, according to Tokukai2001-222838, is a liquid crystal panel in which a circular band oftransparent electrode is formed on a liquid crystal layer filled withbirefringent liquid crystal. The magnitude of correction is varied inaccordance with the voltage applied to the transparent electrode. Theliquid crystal panel as the spherical aberration correction means islocated on the optical axis of a laser producing element. The laser beamradiating from the laser producing element thus develops a phasedifference at its wavefront, passes through it, and converges on therecording surface of an optical disc.

Japanese published patent application 2000-11388 (Tokukai 2000-11388;published on Jan. 14, 2000) discloses a method of correcting sphericalaberration in an optical system in accordance with the magnitude of thespherical aberration, using prerecorded prepit data on an optical discas a reference signal. The spherical aberration is corrected byobserving the amplitude of the reference signal while varying themagnitude of correction so as to produce an final spherical correctionassisting quantity which will be the magnitude of correction at whichthe amplitude assumes a maximum value.

Tokukai 2000-11388, a conventional art, has a problem that it is notapplicable to discs without prepit signals. In addition, the prepitsignal is stored, for example, in sector marks which in general givesuch a small amount of data that the areas may not be sufficient toenable accurate observation of the magnitude of the correction of thespherical aberration. Moreover, in cases of write once, rewriteable, andother writeable optical discs, if the data derived from the prepitsignal is applied to correction for storage areas, accurate correctionis likely to be impossible because of, strictly speaking, differentstorage mechanisms: In prepit areas, recording utilizes the intensity ofreflection which decreases when light diffracts in pit sections. Instorage areas, recording utilizes the presence/absence of an increase inabsorption by storage sections (tint signal).

Japanese published patent application 64-27030/1989 (Tokukaisho64-27030; published on Jan. 30, 1989) provides exemplary write power andfocus offset correction for writeable optical discs. According toTokukaisho 64-27030, focus offset of an optical disc is corrected usingan information readout signal as a reference signal. Write power isvaried from one sector to another. After writing, the sectors aresimultaneously read to find the sector with an optimal result. The writepower for that sector is the optimal write power.

A disadvantage of the Tokukai 2001-222838 method above is an extendedtime the method needs to determine the magnitude of correction, becausethe spherical aberration correction means needs to examine the magnitudeof correction throughout the available range to find a magnitude ofcorrection at which the amplitude of the tracking error signal is at amaximum. Further, at lower spherical aberrations, the amplitude of thetracking error signal changes less, which makes it impossible toaccurately find the maximum amplitude in the presence of noise, externaldisturbance, and other factors.

Tokukaisho 64-27030, a conventional art, has a problem of an extendedtime it takes to write multiple sectors using multiple write powers andread all the sectors to find an optimal magnitude of correction.

An alternative to the prepit signal as the reference signal is a trackcross signal obtained when the optical pickup crosses a track. With thistechnique, however, the signal level can show a maximum value when thespherical aberration and the focus offset are not completely eliminated.If this happens, conversion to an optimal condition cannot be achieved.

This particular problem will be described in more detail in reference toFIG. 18 which is a graph showing measurements of the levels of areference signal in relation to two kinds of parameters, i.e. thespherical aberration and the focus offset. The measurements wereperformed by the inventors of the instant invention. The optical discused contained a 0.1 mm thick transparent, polycarbonate substrate andhas a track pitch of 0.32 μm and a disc groove depth of 21 nm. Thepickup used in the measurement had a laser wavelength of 405 nm. Theobjective lens had a NA of 0.85.

FIG. 18 is a 2-dimensional map of a total of 6×11=66 data points,showing maximum amplitude values of a track cross signal. Six sphericalaberrations from −80 mλ to +80 mλ were plotted on the horizontal axis,and 11 focus offsets from −0.22 to +0.22 μm were plotted on the verticalaxis. “mλ” is a general unit for aberration. “λ” is the wavelength of alaser where 1 mλ=0.001λ. For example, λ=405 nm for a typical blue laser.

As obviously seen from FIG. 18, the reference signal is at a maximum inthe left-falling region immediately surrounding the original point whereeither the spherical aberration or the focus offset is not 0. This showsthat the reference signal can be at a maximum even when the lens-to-lensdistance is not optimal and the lenses are out of focus. The track crosssignal therefore cannot be used for accurate measurement of sphericalaberration and focus offset.

The present invention has an objective to provide a spherical aberrationcorrection method for an optical pickup capable of quick correction ofspherical aberration using an accurate optimal magnitude of aberrationcorrection without being affected by noise, external disturbance, andother factors, and also to provide such an optical pickup.

The present invention has another objective to provide a sphericalaberration focus offset correction method for an optical pickup capableof quick and accurate correction of spherical aberration and focusoffset for a writeable optical disc, and also to provide an opticalpickup with such correction functions.

DISCLOSURE OF INVENTION

An optical pickup of the present invention is an optical pickupprojecting a collected beam onto a recording surface of an opticalstorage medium to retrieve recorded information by means of theintensity of reflection from the recording surface. The pickup correctsa first spherical aberration in an optical system by producing atcorrecting means a second spherical aberration which cancels the firstspherical aberration. The pickup is characterized in that the correctingmeans is capable of producing at least two second spherical aberrationsof different magnitudes by means of a collected beam spot on therecording surface of the optical storage medium so that the magnitudesare ¼ or more of the wavelength λ in P-V values or 1/14 or more of thewavelength λ in standard deviation. The pickup is characterized also inthat the pickup includes control means which: causes the correctingmeans to produce the at least two second spherical aberrations ofdifferent magnitudes; calculates an optimal magnitude of aberrationcorrection for the first spherical aberration through a numericevaluation based on an evaluation value of a reference signal obtainedby receiving the reflection of intensities in the presence of thespherical aberrations of such magnitudes; and controls the correctingmeans to carry out correction using the optimal magnitude of aberrationcorrection.

A method of correcting a spherical aberration of an optical pickup ofthe present invention is a method of correcting a first sphericalaberration in an optical system by producing a second sphericalaberration which cancels the first spherical aberration when the pickupprojects a collected beam onto a recording surface of an optical storagemedium to retrieve recorded information by means of the intensity ofreflection from the recording surface. The method is characterized inthat it involves the steps of: producing at least two second sphericalaberrations of different magnitudes by means of a collected beam spot onthe recording surface of the optical storage medium so that themagnitudes are ¼ or more of the wavelength λ in P-V values or 1/14 ormore of the wavelength λ in standard deviation; calculating an optimalmagnitude of aberration correction for the first spherical aberrationthrough a numeric evaluation based on an evaluation value of a referencesignal obtained by receiving the reflection of intensities in thepresence of the spherical aberrations of such magnitudes; and correctingthe first spherical aberration using the optimal magnitude of aberrationcorrection.

According to the present invention, the second spherical aberrationsproduced by the correcting means are ¼ or more of the wavelength λ inP-V values or 1/14 or more of the wavelength λ in standard deviation. Areference signal is produced by correcting using these second sphericalaberrations and receiving the reflection of intensities from therecording surface of the optical storage medium. In terms of the natureof changes in the evaluation value of the reference signal, a part ofthat signal can be utilized where the changes in the evaluation valueshow high sensitivity to changes in the magnitude of correction.

The optimal magnitude of aberration correction is calculated not basedon the detection of a peak or bottom value, but through a numericevaluation. A numeric evaluation using the second spherical aberrationsand the associated levels of the reference signal, not affected bynoise, external disturbances, or other unwanted factors, will produce asingle optimal magnitude of aberration correction. The detected optimalmagnitude of aberration correction is thus accurate.

In addition, at least two points of measurement are needed where aspherical aberration of the second magnitude is produced and a numericevaluation is performed based on a reference signal. Unlike conventionalcases, there is no need to measure across the entire range of thevariable magnitude of spherical aberration. This allows for an attemptto reduce the time it takes to perform measurement related to sphericalaberration correction.

In the present invention, in the numeric evaluation, the control meansmay calculate an approximation curve from the at least two secondspherical aberrations of different magnitudes produced by the correctingmeans and the evaluation value for these second spherical aberrationsand designate a peak or bottom position of the approximation curve asthe optimal magnitude of aberration correction.

According to the present invention, the evaluation value of an actualreference signal may not have a clear peak or bottom against changes inmagnitude of the spherical aberration correction. In these cases, anapproximation curve can still be calculated to determine a singlevirtual peak or bottom. Thus, a single optimal magnitude of aberrationcorrection can be determined.

In the present invention, the approximation curve may be a multiple termapproximation curve.

According to the present invention, the approximation curve is amultiple term approximation curve. The calculation formulae isrelatively easy. The invention can therefore be implemented oncalculation circuitry of a relatively small scale or calculationsoftware of a relatively small volume.

In the present invention, the control means may be arranged to: causethe correcting means to produce the two second spherical aberrations ofdifferent magnitudes so that the two second spherical aberrations areseparated by ½ or more of the wavelength λ in P-V values and that thesecond spherical aberrations have substantially equal evaluation values;calculate a mean value of the two magnitudes of the sphericalaberrations as the numeric evaluation; and use the mean value obtainedin the mean value calculation as the optimal magnitude of aberrationcorrection.

According to the present invention, the correcting means produces thetwo second spherical aberrations of different magnitudes so that thereference signals obtainable in accordance with the magnitudes ofspherical aberration have substantially equal evaluation values. Thenumeric evaluation produces a mean value of the two second sphericalaberrations of different magnitudes. Since the optimal magnitude ofaberration correction is the mean value of the two second sphericalaberrations of different magnitudes, the optimal magnitude of aberrationcorrection thus calculated is accurate.

In the present invention, the control means may be arranged to: causethe correcting means to produce a second spherical aberration of a firstmagnitude and a second spherical aberration of a second magnitude whichis separated by ½ or more of the wavelength λ in P-V values from thesecond spherical aberration of the first magnitude so that the secondspherical aberration of the second magnitude can produce a referencesignal having an evaluation value substantially equal to that of areference signal obtained in the production of the second sphericalaberration of the first magnitude; calculate a mean value of the secondspherical aberrations of the first and second magnitudes as the numericevaluation; and use the mean value obtained in the mean valuecalculation as the optimal magnitude of aberration correction.

According to the present invention, the correcting means produces thesecond spherical aberration of the first magnitude and the secondspherical aberration of the second magnitude which is separated by ½ ormore of the wavelength λ in P-V values from the second sphericalaberration of the first magnitude so that the second sphericalaberration of the second magnitude can produce a reference signal havingan evaluation value substantially equal to that of a reference signalobtained in the production of the second spherical aberration of thefirst magnitude. The numeric evaluation produces a mean value of the twosecond spherical aberrations. The use of the mean value obtained in themean value calculation renders the calculation of the optimal magnitudeof aberration correction more accurate.

Note that if the optical pickup is characterized in that the referencesignal is an information signal read from the recording surface of theoptical storage medium and also that an evaluation value of thereference signal is an amplitude level, the spherical aberrationcorrection is carried out with increased accuracy, because theinformation signal of which the quality must be ensured by the opticalpickup is designated as the direct reference signal.

If the optical pickup is characterized in that the reference signal is atracking error signal and also that an evaluation value of the referencesignal is an amplitude level, the pickup becomes more immune to noise,external disturbance, and other unwanted factors, because the referencesignal is a tracking error signal where the signal amplitude is largeand sensitive.

If the optical pickup is characterized in that the reference signal isan information signal and also that an evaluation value of the referencesignal is jitter, the spherical aberration correction is carried outwith increased accuracy, because the evaluation value is jitter which ishighly related to information signal quality.

If the optical pickup is characterized in that the reference signal isan information signal and also that an evaluation value of the referencesignal is an error rate, the spherical aberration correction is carriedout with increased accuracy, because the evaluation value is an errorrate which is highly related to information signal quality.

In the present invention, the correcting means may be arranged toinclude: a liquid crystal panel containing a circular band oftransparent electrode provided on a liquid crystal layer filled withbirefringent liquid crystal; and a liquid crystal drive circuit applyingto the transparent electrode voltages corresponding to the at least twosecond spherical aberrations of different magnitudes.

According to the present invention, the liquid crystal layer is placedunder an electric field, and a desired magnitude of spherical aberrationcan be immediately produced with no mechanical motion based on thebirefringent nature of the liquid crystal. Therefore, the magnitude ofspherical aberration can be accurately managed.

In the present invention, the correcting means may be arranged to be abeam expander including a pair of lenses and capable of producing thesecond spherical aberrations by varying a distance between the lenses.

According to the present invention, the beam expander, or the correctingmeans, is relatively easy to build into the optical pickup andcalibrate, because it is not much affected by the relative displacementfrom the beam-collecting objective lens.

In the present invention, the correcting means may be arranged to bepositioned on an optical path along which the beam projected onto therecording surface of the optical storage medium and the reflection fromthe recording surface travel.

According to the present invention, the correction by the magnitude ofspherical aberration produced by the correcting means is done on anoptical path on which the beam projected onto the recording surface ofthe optical storage medium travels and so does the reflection from therecording surface. Therefore, the correction is doublefold on theprojected light and the reflected light. Each magnitude of sphericalaberration result in double correction, equivalent to two times themagnitude of spherical aberration.

In the present invention, the optical pickup may be arranged so that:the control means causes the correcting means to produce a secondspherical aberration of a first magnitude and a second sphericalaberration of a second magnitude so that the second spherical aberrationof the second magnitude can produce a reference signal having anevaluation value substantially equal to that of a reference signalobtained in the production of the second spherical aberration of thefirst magnitude, calculates a mean value of the second sphericalaberrations of the first and second magnitudes as the numericevaluation, and uses the mean value obtained in the mean valuecalculation as the optimal magnitude of aberration correction; and thefirst and second magnitudes are smaller than a maximum signal amplitudeby 5% or more.

According to the present invention, the second spherical aberrations ofthe first and second magnitudes are produced so that they are 5% or moresmaller than a maximum signal amplitude which is a peak amplitude. Theevaluation values of the reference signals are rendered substantiallyequal. The optimal magnitude of aberration correction is calculatedthrough the calculation of a mean value. Therefore, the correction isaccurate.

In the present invention, the optical pickup may be arranged so that:prior to adjustment of a focus offset, the control means: causes thecorrecting means to produce a second spherical aberration of a firstmagnitude and a second spherical aberration of a second magnitude sothat the second spherical aberration of the second magnitude can producea reference signal having an evaluation value substantially equal tothat of a reference signal obtained in the production of the secondspherical aberration of the first magnitude; calculates a mean value ofthe second spherical aberrations of the first and second magnitudes asthe numeric evaluation; and uses the mean value obtained in the meanvalue calculation as the optimal magnitude of aberration correction; andthe first and second magnitudes are smaller than a maximum signalamplitude by 10% or more.

According to the present invention, the second spherical aberrations ofthe first and second magnitudes are produced so that they are 10% ormore smaller than a maximum signal amplitude which is a peak amplitude.The evaluation values of the reference signals are renderedsubstantially equal. The optimal magnitude of aberration correction iscalculated through the calculation of a mean value. Therefore, theoptimal magnitude of aberration correction can be calculated withimproved accuracy, and the resultant correction is accurate.

A method of correcting a spherical aberration focus offset of an opticalpickup of the present invention corrects a spherical aberration and afocus offset in an optical system when the pickup projects a collectedbeam onto a recording surface of an optical storage medium to retrieverecorded information by means of the intensity of reflection from therecording surface. The method is characterized in that it involves: thestep of recording a signal on the storage medium at a predeterminedwrite power; the step of reproducing the recorded information from thereflection; the step of producing a first correction target in thepresence of a predetermined second correction target and changing thefirst correction target, where the first correction target is either oneof the focus offset and the spherical aberration, and the secondcorrection target is the other one; the optimal first correction targetdetection step of detecting an occurrence condition of the firstcorrection target when the first correction target is a minimum; thestep of producing the second correction target under an occurrencecondition of the minimum first correction target and changing themagnitude of the second correction target; and the optimal secondcorrection target detection step of detecting an occurrence condition ofthe second correction target when the second correction target is aminimum, wherein the magnitude of the spherical aberration and themagnitude of the focus offset obtained in the first correction targetdetection step and the optimal second correction target detection stepare used to correct the spherical aberration and the focus offset.

Another method of correcting a spherical aberration focus offset of anoptical pickup of the present invention corrects a spherical aberrationand a focus offset in an optical system when the pickup projects acollected beam onto a recording surface of an optical storage medium toretrieve recorded information by means of the intensity of reflectionfrom the recording surface. The method is characterized in that itinvolves: the step of recording a signal on the storage medium at apredetermined write power; the step of reproducing the recordedinformation from the reflection; the step of producing a sphericalaberration in the presence of a predetermined focus offset and changingthe magnitude of the spherical aberration; the optimal sphericalaberration detection step of detecting a spherical aberration occurrencecondition when the spherical aberration is a minimum; the step ofproducing a focus offset under the minimum spherical aberrationoccurrence condition and changing the magnitude of the focus offset; andthe optimal focus offset detection step of detecting a focus offsetoccurrence condition when the focus offset is a minimum, wherein themagnitude of the spherical aberration and the magnitude of the focusoffset obtained in the optimal spherical aberration detection step andthe optimal focus offset detection step are used to correct thespherical aberration and the focus offset.

Another method of correcting a spherical aberration focus offset of anoptical pickup of the present invention corrects a spherical aberrationand a focus offset in an optical system when the pickup projects acollected beam onto a recording surface of an optical storage medium toretrieve recorded information by means of the intensity of reflectionfrom the recording surface. The method is characterized in that itinvolves: the step of recording a signal on the storage medium at apredetermined write power; the step of reproducing the recordedinformation from the reflection; the step of producing a focus offset inthe presence of a predetermined spherical aberration and changing themagnitude of the focus offset; the optimal focus offset detection stepof detecting a focus offset occurrence condition when the focus offsetis a minimum; the step of producing a spherical aberration under theminimum focus offset occurrence condition and changing the magnitude ofthe spherical aberration; and the optimal spherical aberration detectionstep of detecting a spherical aberration occurrence condition when thespherical aberration is a minimum, wherein the magnitude of thespherical aberration and the magnitude of the focus offset obtained inthe optimal focus offset detection step and the optimal sphericalaberration detection step are used to correct the spherical aberrationand the focus offset.

According to the arrangement, in correcting the spherical aberration andthe focus offset of the optical pickup, the inventors varied twoparameters, the spherical aberration and the focus offset, independentlyand paid attention to a finding that the optimal value of either one ofthe parameters could be obtained without being affected, when the otherparameter was not optimal.

Accordingly, in read operation, the spherical aberration is first variedto detect an optimal magnitude of spherical aberration. Subsequently,using that optimal magnitude of spherical aberration, the focus offsetis varied to detect an optimal magnitude of focus offset. Alternatively,in read operation, the focus offset is first varied to detect an optimalmagnitude of focus offset. Subsequently, using that optimal magnitude ofspherical aberration, the spherical aberration is varied to detect anoptimal magnitude of spherical aberration.

Thus, the spherical aberration and focus offset of a writeable opticaldisc can be corrected quickly and accurately.

The method of correcting a spherical aberration focus offset of anoptical pickup of the present invention is characterized in that aspherical aberration and/or a focus offset are produced which maximizean amplitude of the reproduced signal.

According to the arrangement, since a reproduction signal of therecorded information of which the quality must be ensured by the opticalpickup is directly designated as a reference signal, the resultantcorrection is accurate, needs no complex signal processing, and isimplemented on a simple circuit.

The method of correcting a spherical aberration focus offset of anoptical pickup of the present invention is characterized in that aspherical aberration and/or a focus offset are produced which minimize ajitter of the reproduced signal.

According to the arrangement, since jitter, which is highly related torecorded information quality which must be ensured by the opticalpickup, is designated as a reference signal, the signal processing isrelatively simple and the resultant correction is highly accurate.

The method of correcting a spherical aberration focus offset of anoptical pickup of the present invention is characterized in that aspherical aberration and/or a focus offset are produced which minimizean error rate of the reproduced signal.

According to the arrangement, since an error rate, which is highlyrelated to recorded information quality which must be ensured by theoptical pickup, is designated as a reference signal, the resultantcorrection boasts the highest levels of accuracy and sensitivity.

An optical pickup of the present invention includes a correction deviceproducing a spherical aberration and a focus offset which cancel aspherical aberration and a focus offset in an optical system forcorrection when the pickup projects a collected beam onto a recordingsurface of an optical storage medium to retrieve recorded information bymeans of the intensity of reflection from the recording surface. Thepickup is characterized in that the correction device includes:recording condition detecting means detecting a recording conditionrecorded in advance on the optical storage medium; test write meanstest-writing a predetermined signal in a test write area of the opticalstorage medium under the recording condition detected by the recordingcondition detecting means; and correcting means executing: the processof producing a first correction target in the presence of apredetermined second correction target and changing the first correctiontarget using a reproduction signal from the test write area, where thefirst correction target is either one of the focus offset and thespherical aberration, and the second correction target is the other one;the optimal first correction target detection process of detecting anoccurrence condition of the first correction target when the firstcorrection target is a minimum; the process of producing the secondcorrection target under an occurrence condition of the minimum firstcorrection target and changing the magnitude of the second correctiontarget; the optimal second correction target detection process ofdetecting an occurrence condition of the second correction target whenthe second correction target is a minimum; and the process of using themagnitude of the spherical aberration and the magnitude of the focusoffset obtained in the first correction target detection process and theoptimal second correction target detection process to correct thespherical aberration and the focus offset.

According to the arrangement, to correct the spherical aberration andthe focus offset, the recording condition detecting means first detectsthe recording conditions of the optical storage medium in, for example,lead-in information. According to the recording conditions, the testwrite means test-writes data in the test write area. Based on areproduction signal from that test write area, the correcting meanscorrects the spherical aberration and the focus offset by the foregoingmethod.

Therefore, the resultant optical pickup is capable of quickly andaccurately correcting spherical aberration and focus offset of awriteable optical disc.

The optical pickup of the present invention is characterized in that thecorrecting means is a beam expander including a pair of lenses andmatches a distance between the lenses to the magnitude of the sphericalaberration obtained in the optimal spherical aberration detection step.

According to the arrangement, accuracy requirements in assembly andcalibration are relatively slack, allowing for easy assembly. Inaddition, there is no need to continuously apply a voltage throughoutspherical aberration correcting operation, saving power consumption.

The optical pickup of the present invention is characterized in that thecorrecting means includes: a liquid crystal panel containing a circularband of transparent electrode provided on a liquid crystal layer filledwith birefringent liquid crystal; and a liquid crystal drive circuitapplying to the transparent electrode voltages corresponding to themagnitude of the spherical aberration obtained in the optimal sphericalaberration detection step.

According to the arrangement, there are no movable sections. The pickuppicks up no external disturbances.

Additional objects, advantages and novel features of the invention willbe set forth in part in the description which follows, and in part willbecome apparent to those skilled in the art upon examination of thefollowing or may be learned by practice of the invention.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic block diagram depicting the construction of anoptical pickup which is an embodiment of the present invention.

FIG. 2 is a plan view depicting the structure of a liquid crystal panelused as aberration correction means in the FIG. 1 embodiment.

FIG. 3 is a drawing depicting the structure of a light receiving face ofthe optical sensor in FIG. 1.

FIG. 4 is a graph depicting the relationship between sphericalaberration and a RF level in the FIG. 1 embodiment.

FIG. 5 is a flow chart depicting the steps of a spherical aberrationcorrection subroutine in the FIG. 1 embodiment.

FIG. 6 is a schematic block diagram depicting the construction of anoptical pickup which is another embodiment of the present invention.

FIG. 7 is a graph depicting the relationship between sphericalaberration and a RF level in the FIG. 6 embodiment.

FIG. 8 is a flow chart depicting the steps of a spherical aberrationcorrection subroutine using a beam expander as aberration correctionmeans in the FIG. 6 embodiment.

FIG. 9 is a graph depicting calculated values of wavefront aberrationwhich occurs in the presence of a substrate thickness deviation andfocus offset.

FIG. 10 is a graph depicting measurements of maximum RF signal amplitudechanges in reproducing RF random data recorded under six different setsof conditions.

FIG. 11 is a graph depicting measurements of maximum RF signal amplitudechanges in reproducing RF random data recorded under six different setsof conditions with a remaining focus offset of +0.14 μm.

FIG. 12 is a graph depicting the relationship between sphericalaberration and a RF level in the third embodiment of the presentinvention.

FIG. 13 is a flow chart depicting the steps of a spherical aberrationcorrection subroutine using a beam expander in accordance with the FIG.12 relationship.

FIGS. 14(a), 14(b), in relation to an optical pickup which is anotherembodiment of the present invention, are drawings depicting measurementsof the relationship between spherical aberration and focus offset foundin a reproduced signal which was experimentally written in the absenceof spherical aberration and focus offset.

FIGS. 15(a), 15(b), in relation to an optical pickup which is a furtherembodiment of the present invention, are drawings depicting an exampleset of measurements of the relationship between spherical aberration andfocus offset found in a reproduced signal which was experimentallywritten in the presence of spherical aberration and focus offset.

FIGS. 16(a), 16(b), in relation to an optical pickup which is a stillfurther embodiment of the present invention, are drawings depictinganother example set of measurements of the relationship betweenspherical aberration and focus offset found in a reproduced signal whichwas experimentally written in the presence of spherical aberration andfocus offset.

FIG. 17 is a drawing depicting a flow chart depicting an example set ofsteps correcting spherical aberration and focus offset for an opticalpickup which is an even further embodiment of the present invention.

FIG. 18 is a graph plotted from measurement of relationship betweenspherical aberration and focus offset.

BEST MODE FOR CARRYING OUT THE INVENTION

The following will describe embodiments of the present invention indetail in reference to figures. Members which are equivalent among theembodiments are designated the same reference numbers, and redundantdescription may be omitted.

Embodiment 1

FIG. 1 depicts the construction of an optical pickup which is the firstembodiment of the present invention. In FIG. 1, a pickup 20 emits a readbeam onto an optical disc 40 as an optical storage medium which rotatesas driven by a spindle motor 30, and receives a reflection from the disc40. In the driving, for every revolution of the optical disc 40, thespindle motor 30 generates a revolution signal RT for output to acontrol circuit 50. After shining a read beam onto the optical disc 40and receiving its reflection, the pickup 20 converts that incoming lightto an electric signal for output to a focus error generation circuit 1,a tracking error generation circuit 2, and a RF signal generator circuit3.

The pickup 20 contains a laser producing element 21, a collimating lens22, a beam splitter 23, a quarter-wave plate 24, a liquid crystal panel25, an objective lens 26, a focusing tracking actuator 27, a collectivelens 28, a cylindrical lens 29, and an optical sensor 31. The laserproducing element 21 generates a laser beam at a predetermined lightpower. The laser beam passes through the liquid crystal panel 25 anddelivered to the objective lens 26. The panel 25 corrects sphericalaberration (first spherical aberration) that is caused by an irregularthickness of the transparent substrate of the optical disc 40. Theliquid crystal panel 25, acting as correcting means, is driven inaccordance with a spherical aberration correction signal SA fed from thecontrol circuit 50 which acts as control means to the liquid crystaldriver 4.

FIG. 2 illustrates the structure of the liquid crystal panel 25 as seenfrom the optical axis of the laser beam. As in the figure, the liquidcrystal panel 25 contains a circular, transparent electrode E1, acircular band of transparent electrode E2, and a liquid crystal layer CLfilled with birefringent liquid crystal molecules. The transparentelectrode E1 measures, for example, about 1600 μm in diameter when theobjective lens 26 has a diameter of 3000 μm. The transparent electrodeE2 has an outer diameter of about 2800 μm. The transparent electrodes E1and E2 are positioned so that their center axes are both located on theoptical axis of the laser beam. The transparent electrode E1 is suppliedwith a predetermined voltage fixed at, for example, 2 volts. Thetransparent electrode E2 is supplied with a liquid crystal drive voltageCV from the liquid crystal driver 4. Under these applied voltages, thoseliquid crystal molecules filling the liquid crystal layer CL which arebeneath the circular band area covered with the transparent electrode E2alter their twist angles in accordance with the liquid crystal drivevoltage CV. In the beam spot SPT formed by the laser beam illuminatingthe liquid crystal panel 25, as shown in FIG. 2, a phase differencedevelops between the rays of light passing through the area covered withthe transparent electrode E2 and the rays of light passing through theother areas in accordance with the liquid crystal drive voltage CV. Thatis, the liquid crystal panel 25, in the transmission process, impartssuch a phase difference at the wavefront of the incoming laser beamemitted by the laser producing element 21 before the beam leaves thepanel 25.

Owing to that action, the liquid crystal panel 25 corrects the sphericalaberration (first spherical aberration) that is caused by an irregularthickness of the transparent substrate of the optical disc 40. In thismanner, in the spherical aberration correction by means of the liquidcrystal panel 25, a desired spherical aberration (second sphericalaberration) which cancels the spherical aberration caused by theirregular thickness of the transparent substrate can be immediatelygenerated with no mechanical motion; the magnitude of sphericalaberration generated for correction can be managed accurately. Theobjective lens 26 collects the laser beam coming from the liquid crystalpanel 25 onto a recording track on the recording surface of the opticaldisc 40 as the aforementioned read beam.

As to focusing, the focusing tracking actuator 27 moves the objectivelens 26 in a direction normal to the recording surface of the opticaldisc 40, or so-called focusing trajectory, by an amount which is inaccordance with a focus drive signal F fed via a servo loop switch 5.

As to tracking, the focusing tracking actuator 27 moves the optical axisof the objective lens 24 in the radial direction of the optical disc 40by an amount which is in accordance with a tracking drive signal T fedvia a servo loop switch 6.

FIG. 3 is a drawing showing the light receiving face of the opticalsensor 31. The reflection of the read beam shone onto the recordingtrack of the optical disc 40 passes through the objective lens 26, theliquid crystal panel 25, and the quarter-wave plate 24, before changingits traveling direction at the beam splitter 23. The light furtherpasses through the collective lens 28 and the cylindrical lens 29, andhits the light receiving face of the optical sensor 31. The opticalsensor 31 has four independent light receiving elements A to D which arearranged as shown in the figure with respect to the track direction. Thelight receiving elements A to D receive reflection from the optical disc40 and convert it to respective electric signals, or photoelectricconverted signals RA to RD, for output.

The focus error generation circuit 1 adds the outputs of the lightreceiving elements A to D which are located at diagonal positions acrossthe optical sensor 31 to produce two sums. The circuit 1 then suppliesthe difference between the sums (focus error signal FE) to thesubtracter 7. In other words, the focus error generation circuit 1supplies a focus error signal FE=(RA+RC)−(RB+RD) to the subtracter 7.The subtracter 7 subtracts a position-on-focusing-trajectory signal FPfrom the focus error signal FE and feeds a resulting focus error signalFE′ to the servo loop switch 5. The subtracter 7 is fed with the signalFP by the control circuit 50. The servo loop switch 5 turns on/off inaccordance with a focus servo switching signal FS fed from the controlcircuit 50.

For example, the servo loop switch 5 turns off when the focus servoswitching signal FS is a logic 0 indicating a focus servo turn-off. Whenthe focus servo switching signal FS is a logic 1 indicating a focusservo turn-on, the switch 5 turns on and starts supplying a focus drivesignal F to the focusing tracking actuator 27 in accordance with thefocus error signal FE′. In other words, a system including the pickup20, the focus error generation circuit 1, the subtracter 7, and theservo loop switch 5 constitutes a so-called focus servo loop. Owing tothis focus servo loop, the objective lens 26 maintains its position onthe focusing trajectory in accordance with theposition-on-focusing-trajectory signal FP.

The tracking error generation circuit 2 adds the outputs of the lightreceiving elements A to D of the optical sensor 31 which are locatedadjacent to each other in the track direction to produce two sums. Thecircuit 2 then supplies the difference between the sums (tracking errorsignal) to the servo loop switch 6. In other words, the circuit 2supplies a tracking error signal (RA+RD)−(RB+RC) to the switch 6. Theservo loop switch 6 turns on/off in accordance with a tracking servoswitching signal TS fed from the control circuit 50.

For example, when the tracking servo switching signal TS is a logic 1indicating a tracking servo turn-on, the servo loop switch 6 turns onand starts supplying a tracking drive signal T to the focusing trackingactuator 27 in accordance with the tracking error signal. When thetracking servo switching signal TS is a logic 0 indicating a trackingservo turn-off, the switch 6 turns off, suspending the supplying of thetracking drive signal T to the focusing tracking actuator 27.

The RF signal generator circuit 3 adds the photoelectric convertedsignals RA to RD to produce a sum (information readout signal)representing information data on the optical disc 40. The circuit 3 thenfeeds the information readout signal to a RF demodulator circuit 10 andthe control circuit 50. The RF demodulator circuit 10 performspredetermined demodulation on the information readout signal to recoverthe information data (RF data representing reproduced information) foroutput.

FIG. 4 shows the RF level of the information readout signal changingwith spherical aberration correction when the FIG. 1 optical pickup hasspherical aberration left uncorrected. Plotting the P-V value of thespherical aberration of the device on the horizontal axis and the RFlevel of the information readout signal on the vertical axis, the RFlevel is a maximum when the spherical aberration is 0. The RF levelhowever hardly changes where the spherical aberration is less than orequal to a reference value for optical characteristics evaluationrelative to a 0 spherical aberration point. Well-known reference valuesfor evaluation are the Rayleigh limit and Strehl Definition. TheRayleigh limit defines a maximum value of wavefront aberration at λ/4 orless. Strehl Definition, or “SD,” defines the standard deviation ofwavefront aberration at λ/14 or less. λ is the wavelength of the lightsource. Collected beams in these cases will be safely regarded as ideal.The P-V value refers to a maximum of the absolute value, that is, amaximum when the value is positive and a minimum when the value isnegative.

In the present embodiment, to detect a magnitude of aberrationcorrection which results in a maximum RF level, sampling points areprovided in a region where the RF level changes greatly with themagnitude of spherical aberration correction. In the figure, foursampling points (SA1 to SA4) are provided as an example. Anapproximation curve L1 is then computed using computing circuitry todefine the magnitude of aberration correction at which the approximationcurve L1 reaches the peak, that is, the optimal magnitude of aberrationcorrection SABEST. When the actually RF signal does not show a distinctpeak or bottom, this technique is still capable of determining a singlevirtual peak or bottom, hence a single optimal magnitude of aberrationcorrection SABEST.

The sampling points may be more or less than four. The approximationcurve L1 can be computed with at least two points. With less samplingpoints involved, computing becomes easier, the computing circuitryarrangement becomes less complicated, and the magnitude of correction isobtained more quickly. With more sampling points involved, theapproximation curve L1 becomes more accurate, and the magnitude ofaberration correction is more accurately observed.

Calculating the approximation curve L1 by multiple term approximationrequires relatively easy formulae, and is implemented by relativelysmall-scale computing circuitry or software. Various interpolations,including spline interpolation, are also available. Spline interpolationwould produce a fewer errors. The computing circuitry that calculatesthe approximation curve L1 and the magnitude of aberration correctionmay be provided by a microcomputer which is programmed to perform thecircuitry's functions, a DSP (digital signal processor) whichincorporates dedicated computing functions, or analog circuitry.

The sampling points may be determined based on a predetermined, recordedmagnitude of correction. Alternatively, the RF level may be observedwhile varying the magnitude of correction. When the RF level matches aprerecorded, target sampling RF value, the magnitude of correctionproducing that RF value is sampled.

Note that the foregoing description assumes the RF level as thereference as an example. Alternatively, the reference may be thetracking error level. When this is the case, aberration is correctedmore accurately, which would be useful in directly employing the signallevel of which the quality needs be ensured by the optical pickup as anevaluation value of the reference signal. Further, the tracking errorsignal, i.e. the reference signal, has a great amplitude and highsensitivity; therefore, the aberration correction is more resistant tonoise and other external disturbances. Further, one can use theinformation signal as the reference signal, and jitter or BER (bit errorrate), which is highly related to information signal quality, as theevaluation value of the reference signal. In this case, accuratespherical aberration is again possible.

FIG. 5 illustrates an example of specific procedures up to thedetermination of the magnitude of spherical aberration correction in thepresent embodiment. The control circuit 50, or control means, performs acontrol operation in accordance with a main routine omitted in thefigure so that the optical information read device can perform variousread/write operations. If the optical disc 40 is inserted in the opticalinformation read device while the control circuit 50 is executing themain routine, the circuit 50 switches its operation to a sphericalaberration correction subroutine. Procedures of the subroutine are shownin FIG. 5. The control circuit 50 stores, in a built-in register,spherical aberration correction signals SA(1) to SA(4) corresponding tothe magnitudes of spherical aberration at the aforementioned foursampling points.

In first step S1, the control circuit 50 supplies a focus servoswitching signal FS representing a logic 1 to the servo loop switch 5 toturn on the focus servo. In next step S2, the control circuit 50supplies a tracking servo switching signal TS representing a logic 0 tothe servo loop switch 6 to turn off the tracking servo, andsets/initialize a variable N to 1.

In step S3, the control circuit 50 retrieves the spherical aberrationcorrection signal SA(N) from the built-in register for output to theliquid crystal driver 4. As a result of step S3, the liquid crystaldriver 4 generates a liquid crystal drive voltage CV in accordance withthe value represented by the spherical aberration correction signalSA(N) for output to the liquid crystal panel 25. Therefore, when theliquid crystal panel 25 illuminated by a laser beam, a phase differencedevelops between the rays of light passing through the area covered withthe circular band of transparent electrode E2 as shown in FIG. 2 and therays of light passing through the other areas in accordance with thespherical aberration correction signal SA(N). The spherical aberrationis hence tentatively corrected.

In next step S4, the control circuit 50 determines, based on therevolution signal RT from the spindle motor 30, whether the optical disc40 has completed one revolution. The determination is repeated until theoptical disc 40 completes a revolution. If the optical disc 40 isdetermined in step S4 to have completed a revolution, the controlcircuit 50 in step S5 acquires a RF signal as RF(N). Next, in step S6,the control circuit 50 determines whether N=4. If it is determined inthe step that N is not equal to 4, the control circuit 50 proceeds tostep S7 where N is incremented by 1 before returning to step S3 wherethe circuit 50 retrieves the spherical aberration correction signalSA(N) from the built-in register. Steps S3 to S7 are repeated.

Steps S3 to S6 are repeated four times while the magnitude of sphericalaberration correction by the liquid crystal panel 25 is being changedfor every repetition, for example, from SA(1) to SA(4). The sphericalaberration correction signals SA(1), SA(2), SA(3), SA(4), eachrepresenting a magnitude of correction, are preferably those producinggreat changes in the RF level. Provided that two of the four points areclose to a maximum magnitude of correction, the other two close to aminimum magnitude of correction, and the maximum magnitude of correctionis divided by 16, SA(1) will be designated to be equivalent to 1/16 ofthe maximum magnitude of correction, SA(2) to 2/16, SA(3) to 15/16, andSA(4) to 16/16.

Next, if the control circuit 50 determines in step S6 that N=4, thecircuit 50 proceeds to step S8 where the circuit 50 determines theoptimal magnitude of aberration correction SABEST giving a maximum RFsignal level RFMAX on an approximation curve. The control circuit 50computes the approximation curve from sampling data, that is, the fourmagnitudes of spherical aberration correction corresponding to thespherical aberration correction signals SA(1) to SA(4) and the RFsignals RF(1) to RF(4) each resulting from a different one of the fourmagnitudes of spherical aberration correction. In next step S9, thecontrol circuit 50 then feeds a signal representing the optimalmagnitude of spherical aberration correction SABEST, or final sphericalaberration correction signal, to the liquid crystal driver 4.

In other words, step S9 specifies the optimal magnitude of sphericalaberration correction SABEST as the final magnitude of sphericalaberration correction and drives the liquid crystal panel 25 so as togive the rays passing through the area covered with the transparentelectrode E2 in FIG. 2 a phase difference in accordance with thismagnitude of correction. The driving performs the final sphericalaberration correction. Following step S9, the control circuit 50 leavesthis spherical aberration correction subroutine to return to the mainroutine. This routine enables accurate determination of an optimalmagnitude of spherical aberration correction at short detection time foraccurate correction.

The spherical aberration correction signal SA may be retrieved for anynumber of times, although it is retrieved four times in the operation inFIG. 5. Further, in the present embodiment, the amplitude of the RFsignal is used in various processing; the amplitude of the trackingerror signal or the servo gain of the tracking servo may be used inplace of the amplitude of the RF signal.

Also, the liquid crystal panel 25 may have concentric circular rings oftransparent electrodes, instead of the single circular band oftransparent electrode E2 in FIG. 2, on its liquid crystal layer CL. Therings of transparent electrodes will provide areas of differing phasedifferences in accordance with the spherical aberration which may differnear the circumference and near the center under the beam spot, which inturn enables elaborate spherical aberration correction. The voltagesapplied to the rings of transparent electrodes are weighted according tothe spherical aberration pattern.

Embodiment 2

The following will describe the second embodiment of the presentinvention in detail with reference to figures. FIG. 6 illustrates theconstruction of an optical pickup which is the second embodiment of thepresent invention. In FIG. 6, a pickup 20 emits a read beam onto anoptical disc 40 as an optical storage medium which rotates as driven bya spindle motor 30, and receives a reflection from the disc 40. In thedriving, for every revolution of the optical disc 40, the spindle motor30 generates a revolution signal RT for output to a control circuit 50.After shining a read beam onto the optical disc 40 and receiving itsreflection, the pickup 20 converts that incoming light to an electricsignal for output to a focus error generation circuit 1, a trackingerror generation circuit 2, and a RF signal generator circuit 3.

The pickup 20 contains a laser producing element 21, a collimating lens22, a beam splitter 23, a quarter-wave plate 24, a beam expander 35, abeam expander actuator 34, an objective lens 26, a focusing trackingactuator 27, a collective lens 28, a cylindrical lens 29, and an opticalsensor 31. The laser producing element 21 generates a laser beam at apredetermined light power. The laser beam enters the beam expander 35which is provided to correct spherical aberration caused by theirregular thickness of the transparent substrate of the optical disc 40.

The beam expander 35 is, for example, a beam-expanding relay lenscontaining a pair of a concave lens 33 and a convex lens 32. Theexpander 35 is adapted normally to receive incident parallel rays oflight and projects parallel rays with an expanded beam diameter. Theconcave lens 33 and the convex lens 32 changes the distance separatingthe two lenses so as to either diverge or converge the light hitting theobjective lens 26 which hence produces spherical aberration. With theseactions, the beam expander 35 can function as correcting means whichcorrects the spherical aberration caused by the irregular thickness ofthe transparent substrate of the optical disc 40. The beam expander 35and the objective lens 26, if not correctly positioned relative to eachother, little affects the occurrence of spherical aberration, and thusrequire only relatively easy adjustment when integrated into an opticalpickup.

The objective lens 26 collects the laser beam coming from the beamexpander 35 onto a recording track on the recording surface of theoptical disc 40 as the aforementioned read beam. As to focusing, thefocusing tracking actuator 27 moves the objective lens 26 in a directionnormal to the recording surface of the optical disc 40, or so-calledfocusing trajectory, by an amount in accordance with a focus drivesignal F fed via a servo loop switch 5. As to tracking, the focusingtracking actuator 27 moves the optical axis of the objective lens 24 inthe radial direction of the optical disc 40 by an amount which is inaccordance with a tracking drive signal T fed via a servo loop switch 6.

The reflection of the read beam illuminating the recording track of theoptical disc 40 travels through the objective lens 26, the beam expander35, and the quarter-wave plate 24, diverts in the beam splitter 23, andtravels on through the collective lens 28 and the cylindrical lens 29before hitting the light receiving face of the optical sensor 31 whichis depicted in FIG. 3.

Referring to FIG. 3, the optical sensor 31 has four independent lightreceiving elements A to D which are arranged as shown in the figure withrespect to the track direction. The light receiving elements A to Dreceive reflection from the optical disc 40 and convert it to respectiveelectric signals, or photoelectric converted signals RA to RD, foroutput. The focus error generation circuit 1 adds the outputs of thelight receiving elements A to D which are located at diagonal positionsacross the optical sensor 31 to produce two sums. The circuit 1 thensupplies the difference between the sums (focus error signal FE) to thesubtracter 7. In other words, the focus error generation circuit 1supplies a focus error signal FE=(RA+RC)−(RB+RD) to the subtracter 7.The subtracter 7 subtracts a position-on-focusing-trajectory signal FPfrom the focus error signal FE and feeds a resulting focus error signalFE′ to the servo loop switch 5. The subtracter 7 is fed with the signalFP by the control circuit 50. The servo loop switch 5 turns on/off inaccordance with a focus servo switching signal FS fed from the controlcircuit 50.

For example, the servo loop switch 5 turns off when the focus servoswitching signal FS is a logic 0 indicating a focus servo turn-off. Whenthe focus servo switching signal FS is a logic 1 indicating a focusservo turn-on, the switch 5 turns on and starts supplying a focus drivesignal F to the focusing tracking actuator 27 in accordance with thefocus error signal FE′. In other words, a system including the pickup20, the focus error generation circuit 1, the subtracter 7, and theservo loop switch 5 constitutes a so-called focus servo loop. Owing tothis focus servo loop, the objective lens 26 maintains its position onthe focusing trajectory in accordance with theposition-on-focusing-trajectory signal FP.

The tracking error generation circuit 2 adds the outputs of the lightreceiving elements A to D of the optical sensor 31 which are locatedadjacent to each other in the track direction to produce two sums. Thecircuit 2 then supplies the difference between the sums (tracking errorsignal) to the servo loop switch 6. In other words, the circuit 2supplies a tracking error signal (RA+RD)−(RB+RC) to the switch 6. Theservo loop switch 6 turns on/off in accordance with a tracking servoswitching signal TS fed from the control circuit 50.

For example, when the tracking servo switching signal TS is a logic 1indicating a tracking servo turn-on, the servo loop switch 6 turns onand starts supplying a tracking drive signal T to the focusing trackingactuator 27 in accordance with the tracking error signal. When thetracking servo switching signal TS is a logic 0 indicating a trackingservo turn-off, the switch 6 turns off, suspending the supplying of thetracking drive signal T to the focusing tracking actuator 27.

The RF signal generator circuit 3 adds the photoelectric convertedsignals RA to RD to produce a sum (information readout signal)representing information data on the optical disc 40. The circuit 3 thenfeeds the information readout signal to a RF demodulator circuit 10 andthe control circuit 50. The RF demodulator circuit 10 performspredetermined demodulation on the information readout signal to recoverthe information data (RF data representing reproduced information) foroutput.

FIG. 7 shows the RF level of the information readout signal changingwith spherical aberration correction when the FIG. 6 optical pickup hasspherical aberration left uncorrected. The spherical aberration iscorrected by changing the lens distance of the beam expander 35.Plotting the P-V value of the spherical aberration of the device on thehorizontal axis and the RF level of the information signal on thevertical axis, the RF level is a maximum when the spherical aberrationis 0. The RF level however hardly changes where the spherical aberrationis less than or equal to a reference value for optical characteristicsevaluation. Well-known reference values for evaluation are the Rayleighlimit and Strehl Definition. The Rayleigh limit defines a maximum valueof wavefront aberration at λ/4. Strehl Definition, or “SD,” defines thestandard deviation of wavefront aberration at λ/14 or less. λ is thewavelength of the light source. Collected beams in these cases will besafely regarded as ideal.

In the present embodiment, to detect a lens distance which results in amaximum RF level, sampling points are provided in a region where the RFlevel changes greatly with the magnitude of spherical aberrationcorrection. In the figure, four sampling points (SP1 to SP4) areprovided as an example. An approximation curve L2 is then computed usingcomputing circuitry to define a lens distance at which the approximationcurve L2 reaches the peak, that is, an optimal lens distance SPBESTresulting in an optimal aberration correction. The sampling points maybe more or less than four. The approximation curve L2 can be computedwith at least two points. With less sampling points involved, computingbecomes easier, the computing circuitry arrangement becomes lesscomplicated, and the magnitude of correction is obtained more quickly.With more sampling points involved, the approximation curve L2 becomesmore accurate, and the lens distance is more accurately observed.

The approximation curve L2 may be computed by various interpolationtechniques, including multiple term approximation and splineinterpolation. The computing circuitry that calculates the approximationcurve L2 and the lens distance may be provided by a microcomputer whichis programmed to perform the circuitry's functions, a DSP (digitalsignal processor) which incorporates dedicated computing functions, oranalog circuitry.

The sampling points may be determined based on a predetermined, recordedlens distance. Alternatively, the RF level may be observed while varyingthe lens distance. When the RF level matches a prerecorded, targetsampling RF value, the lens distance producing that RF value is sampled.

Note that the foregoing description assumes the RF level as theevaluation reference as an example. Alternatively, the reference may bethe tracking error level, jitter, or BER (bit error rate) as theevaluation reference.

FIG. 8 illustrates an example of specific procedures up to thedetermination of the lens distance for spherical aberration correctionin the present embodiment. The control circuit 50, or control means,performs a control operation in accordance with a main routine omittedin the figure so that the optical information read device can performvarious read/write operations. If the optical disc 40 is inserted in theoptical information read device while the control circuit 50 isexecuting the main routine, the circuit 50 switches its operation to aspherical aberration correction subroutine. Procedures of the subroutineare shown in FIG. 8. The control circuit 50 stores, in a built-inregister, lens distance signal SP(1) to SP(4) corresponding to the lensdistance SP1 to SP4 at the aforementioned four sampling points.

In step S11, the control circuit 50 supplies a focus servo switchingsignal FS representing a logic 1 to the servo loop switch 5 to turn onthe focus servo. In next step S12, the control circuit 50 supplies atracking servo switching signal TS representing a logic 0 to the servoloop switch 6 to turn off the tracking servo, and sets/initialize avariable N to 1.

In step S13, the control circuit 50 retrieves the lens distance signalSP(N) from the built-in register for output to the beam expander driveactuator 34. As a result of step S3, the beam expander drive actuator 34drives the beam expander 35 so that the lens distance is in accordancewith the value of the lens distance signal SP(N). Therefore,non-parallel light hits the objective lens 26, producing sphericalaberration in accordance with the lens distance signal SP(N). Thespherical aberration is hence tentatively corrected.

In next step S14, the control circuit 50 determines, based on therevolution signal RT from the spindle motor 30, whether the optical disc40 has completed one revolution. The determination is repeated until theoptical disc 40 completes a revolution. In next step S15, the controlcircuit 50 acquires a RF signal level as RF(N). Next, in step S16, thecontrol circuit 50 determines whether N=4. If it is determined in thestep that N is not equal to 4, the control circuit 50 proceeds to stepS17 where N is incremented by 1 before returning to step S13 where thecircuit 50 retrieves the lens distance signal SP(N) from the built-inregister to tentatively correct the spherical aberration. Steps S13 toS17 are repeated.

Steps S13 to S17 are repeated four times while the lens distance of thebeam expander 35 producing spherical aberration correction is beingchanged for every repetition, for example, in accordance with the lensdistance signals SP(1) to SP(4). The lens distance signals SP(1), SP(2),SP(3), SP(4), each representing a magnitude of correction, arepreferably those producing great changes in the RF level. Provided thattwo of the four points are close to a maximum lens distance, the othertwo close to a minimum lens distance, and the signal magnitudecorresponding to the maximum lens distance is divided by 16, SP(1) willbe designated to be equivalent to 1/16 of the maximum lens distance,SP(2) to 2/16, SP(3) to 15/16, and SP(4) to 16/16.

If the control circuit 50 determines in step S16 that N=4, the circuit50 proceeds to step S18 where the circuit 50 determines the optimal lensdistance SPBEST giving a maximum RF signal level RFMAX on theapproximation curve L2 in FIG. 7. The control circuit 50 computes theapproximation curve L2 from sampling data, that is, the four lensdistances SP1 to SP4 corresponding to the lens distance signals SP(1) toSP(4) and the RF signal levels RF1 to RF4 each resulting from adifferent one of the four lens distances. In step S19, the controlcircuit 50 then feeds a signal representing the optimal lens distanceSPBEST, or a lens distance signal for final spherical aberrationcorrection, to the beam expander drive actuator 34. In other words, stepS19 specifies the lens distance corresponding to the optimal lensdistance SPBEST as the final lens distance and gives the objective lens26 a spherical aberration of which the amount is in accordance with thelens distance, so as to cancel the spherical aberration caused by theirregular thickness of the transparent substrate of the optical disc.The step performs the final spherical aberration correction. Followingstep S19, the control circuit 50 leaves this spherical aberrationcorrection subroutine to return to the main routine.

This routine enables accurate determination of an optimal magnitude ofspherical aberration correction at short detection time for accuratecorrection. The lens distance signal SP(N) may be adjusted for anynumber of times, although it is adjusted four times in the operation inFIG. 8. Further, in the present embodiment, the amplitude of the RFsignal is used in various processing; the amplitude of the trackingerror signal or the servo gain of the tracking servo may be used inplace of the amplitude of the RF signal.

In the beam expander 35 in FIG. 6, the concave lens 33, which is thesmaller of the two lenses, is moved. This structure has effectivelyreduced the driving power and physical size of the beam expander driveactuator 34. The convex lens 32 or both lenses may be moved. The beamexpander 35 provides an expanding optical system with the concave lens33 and the convex lens 32 being placed in this order in the opticalpath. Alternatively, the convex lens 32 and the concave lens 33 may beplaced in this order to form a narrowing optical system.

The optical pickup tolerance for the wavefront aberration (standarddeviation) is λ/14. However, the tolerance for the spherical aberrationis preferably about 35 mλ in the present embodiment when the followingcontributions to the overall wavefront aberration, other than thatcaused by the thickness irregularity of the transparent substrate, arealso considered: namely, the wavefront aberration caused by the opticalcomponents in the pickup 20 itself, the wavefront aberration caused by atilting disc, and the aberration upon defocusing which occurs with afocus offset remaining when the readout from the optical disc 40 issubjected to focus servo.

Specific examples will be given now. FIG. 9 shows simulation ofwavefront aberration in the presence of substrate thickness deviationsand focus offsets. The objective lens NA (numerical aperture) is set to0.85, the transparent substrate thickness in an ideal disc to 0.1 mm,and the beam scale up ratio of the beam expander 35 to 1.5. Thesubstrate thickness deviation is plotted in μm on the horizontal axis.The F (focus) offset is plotted in μm on the vertical axis. Thewavefront aberration values are indicated in rms, relative to thewavelength λ. The figure tells that the thickness deviation which cantolerate a wavefront aberration of 35 mλ or less is about −3.5 to 3 μmwhen the focus offset is 0.

FIG. 10 shows measurements of maximum RF signal amplitude changes inreproducing RF random data recorded under the following six differentsets of conditions. The horizontal axis indicates a substrate thicknessdeviation equivalent of the wavefront aberration produced by the beamexpander (BE) 35 in the reproduction operation. A sampling interval isequivalent to a substrate thickness deviation of about 1.5 μm. (Forexample, a CG thickness error of 1 μm is equivalent to a wavefrontaberration produced by the beam expander 35 when the substrate thicknessis 1 μm thicker than designed.)

Recording Conditions:

(i) Optimal write power at zero aberration and zero F offset (B0F0).

(ii) Write power 20% more than optimal at zero aberration and zero Foffset (B0F0).

(iii) Write power 20% less than optimal at zero aberration and zero Foffset (B0F0).

(iv) Optimal write power at aberration equivalent to 7 μm thicknessirregularity of a transparent substrate and F offset of −0.1 μm (B7F1).

(v) Write power 20% more than optimal at aberration equivalent to 7 μmthickness irregularity of a transparent substrate and F offset of −0.1μm (B7F1).

(vi) Write power 20% less than optimal at aberration equivalent to 7 μmthickness irregularity of a transparent substrate and F offset of −0.1μm (B7F1).

Now, assuming the foregoing tolerable thickness deviation of −3.5 to 3μm and a thickness deviation between individual optical discs 40 of ±2μm, the optical pickup can tolerate spherical aberration equivalent to athickness deviation of −1 to 1.5 μm. Further, specifying a margin thatis equivalent to the thickness deviation of ±0.5 μm as a controlledpositional error of the beam expander 35, the tolerable adjustment errorfor the beam expander 35 is −1 to 0.5 μm.

As shown in the figure, for each data set obtained under the six sets ofrecording conditions, a ◯ indicates the midpoint between the samplingpoints, on either side of the sampling point representing a peakamplitude, at which the amplitude drops 3% or more for the first timefrom the peak; a Δ indicates the midpoint between the sampling points,on either side of the sampling point representing a peak amplitude, atwhich the amplitude drops 5% or more for the first time from the peak;and a □ indicates the midpoint between the sampling points, on eitherside of the sampling point representing a peak amplitude, at which theamplitude drops 10% or more for the first time from the peak. Some ◯points fall out of the tolerable adjustment error range, −1 to 0.5 μm,for the beam expander 35. The other points, Δ and □, are within thattolerable range. Accordingly, suitable RF levels for sampling are atleast 5% less than the peak amplitude. When this is the case, the beamexpander 35 can be adjusted more accurately irrespective of therecording conditions of the reproduced data.

FIG. 11 shows measurements of maximum RF signal amplitude changes inreproducing RF random data recorded under the foregoing six differentsets of conditions. There is a remaining focus offset of +0.14 μm. It isassumed that the beam expander (BE) distance is adjusted before theadjustment of the focus offset. Similarly to the previous figure, a Δpoint indicates the midpoint between the sampling points, on either sideof the sampling point representing a peak amplitude, at which theamplitude drops 5% or more for the first time from the peak; and a □point indicates the midpoint between the sampling points, on either sideof the sampling point representing a peak amplitude, at which theamplitude drops 10% or more for the first time from the peak. Some Δpoints fall out of the tolerable adjustment error range, −1 to 0.5 μm,for the beam expander 35. The other, □ points are within that tolerablerange. Accordingly, if there remains a focus offset, suitable RF levelsfor sampling are at least 10% less than the peak amplitude. When this isthe case, the beam expander 35 can be adjusted more accuratelyirrespective of the recording conditions of the reproduce data.

Embodiment 3

Next, a third embodiment of the present invention will be described indetail in reference to figures.

FIG. 12 shows the RF level changing with changes of the lens distance ofa beam expander 35 in spherical aberration correction. The beam expander35 here is in an optical pickup of the present embodiment which isarranged similarly to the FIG. 6 embodiment. The optical pickup has aspherical aberration left uncorrected (first spherical aberration). InFIG. 12, similarly to FIG. 7, the spherical aberration of the device isplotted on the horizontal axis, and the RF level of an informationsignal is plotted on the vertical axis. The RF level is a maximum atzero spherical aberration. The RF level hardly changes at aberrationsequal to or less than a reference value for optical characteristicsevaluation. Well-known reference values for evaluation are, as mentionedearlier, the Rayleigh limit (defining a maximum value of wavefrontaberration at λ/4 or less) and SD (Strehl Definition, defining thestandard deviation of wavefront aberration at λ/14 or less). In thesecases, we regard the collected beam as being almost ideal.

In the present embodiment, to detect the lens distance when the RF levelis a maximum, the lenses are moved to a lens distance SP1 close to aminimum setting in the movable range. Letting the RF level at SP1 be P,the lens distance is gradually increased to find a lens distance SP2 atwhich the RF level returns to P. Since the lens distances SP1, SP2 fallout of the Rayleigh limits of ±λ/4, the lens distance is greater thanλ/2. The optimal lens distance SPBEST for optimal aberration correctionis set to (SP1+SP2)/2. According to the method, the optimal lensdistance SPBEST is equal to the mean value of the two samplings. Thisallows for simplification and downsizing of computing circuitry.

This approach is an example where the lens distance SP1 ispredetermined. Another approach is to use a predetermined RF level P anddetect the lens distances SP1, SP2 at which the RF level equals P. Inthe latter approach, the lens distance can be detected accurately in thepresence of irregular RF level sensitivity between individual devices.

The RF level is not the only possible evaluation reference. The trackingerror level, jitter, and BER (bit error rate) are other candidates forevaluation reference.

FIG. 13 illustrates specific procedures for determining the lensdistance in the present embodiment, as an example. The control circuit50 executes control operation on the optical pickup in accordance with amain routine (not shown) to implement various read/write actions. If theoptical disc 40 is inserted in the optical pickup while the controlcircuit 50 is executing the main routine, the control circuit 50proceeds to execute a spherical aberration correction subroutine for thebeam expander 35. The subroutine is shown in steps in FIG. 13.

Referring to FIG. 13, in first step S21, the control circuit 50 suppliesa “logic 1” focus servo switching signal FS to the servo loop switch 5to turn on the focus servo. In next step S22, the control circuit 50supplies a “logic 0” tracking servo switching signal TS to the servoloop switch 6 to turn off the tracking servo. The circuit 50 alsoinitializes N=1.

In step S23, the control circuit 50 outputs a drive signal so that thebeam expander drive actuator 34 moves the lenses of the beam expander 35to a lens distance in accordance with the lens distance signal SP(N). Asstep S23 is executed, non-parallel light enters the objective lens 26,producing a spherical aberration in accordance with the lens distancesignal SP(N). The spherical aberration is thus tentatively corrected. Innext step S24, the control circuit 50 determines whether the opticaldisc 40 has completed one revolution, based on a revolution signal RTfed from the spindle motor 30, and repeats the process until the opticaldisc 40 completes one revolution. In next step S25, the control circuit50 acquires the RF signal level and assigns it RF(N). In next step S26,the control circuit 50 determines whether the RF(N) equals thepredetermined value P. If RF(N) is not determined to equal P in thisstep, the control circuit 50 proceeds to step S27 where N is incrementedby 1 and returns to step S23. In step S23, the circuit 50 retrieves thelens distance signal SP(N) from the built-in register and carries outsampling. Steps S23 to S27 above are repeated.

If RF(N) is determined to equal P, in step S26, the control circuit 50proceeds to step S28 to determine whether the RF(N) has reached P forthe first time. If RF(N) is determined in step S28 to have reached P forthe first time, the control circuit 50 proceeds to step S29 where itstores SP(N) under the name of SP1. In step S30, the control circuit 50increments N by 1 and returns to step S23 to retrieve the lens distancesignal SP(N) from the built-in register. Step S23 to S27 above arerepeated.

If in step S28 it is determined that RF(N) has reached P for the secondtime, the control circuit 50 proceeds to step S31 where it stores SP(N)under the name of SP2. In next step S32, the optimal lens distanceSPBEST=(SP1+SP2)/2 is obtained. In step S33, the control circuit 50supplies the optimal lens distance signal SPBEST to the beam expanderdrive actuator 34 as a lens distance signal for a final sphericalaberration correction. In other words, the execution of step S33 obtainsthe lens distance corresponding to the optimal lens distance signalSPBEST as a final lens distance and provides the objective lens 26 aspherical aberration in accordance with the lens distance to cancel thespherical aberration produced by the irregular thickness of thetransparent substrate of the optical disc (final spherical aberrationcorrection). After step S33, the control circuit 50 leaves the sphericalaberration correction subroutine to return to the main routine. Thisroutine enables accurate determination of an optimal magnitude ofspherical aberration correction at short detection time for accuratecorrection.

In the process illustrated in FIG. 13, the lens distance signal SP isallowed to change between the minimum and maximum distances. Thatminimum-to-maximum range may be divided into, for example, 16 equalintervals within the lens distance resolution capability of the beamexpander so that the beam expander can change the lens distance at thatinterval or less to find SP1 and SP2. When this is the event, a limit isimposed on the number of times the lens distance detection is carriedout. The spherical aberration correction subroutine is never repeatedexceeding that limit, thus quickly completing the spherical aberrationcorrection.

The present embodiment utilizes the RF signal amplitude in variousprocessing. In place of the RF signal amplitude, a tracking error signalamplitude or a tracking servo gain may be used.

As shown in FIG. 4, FIG. 7, and FIG. 12, the embodiments above evaluatemagnitude of correction through a value having a peak which will beregarded as giving the optimal magnitude of correction. The RF signalamplitude is an example of such a value. The embodiments will also workwith a value with a bottom giving the optimal magnitude of correction.

Embodiment 4

The present embodiment relates to spherical aberration focus offsetcorrection method for an optical pickup used in optical read/writedevices capable of reading/writing high density write once, rewriteable,and like optical discs, as well as to an optical pickup with such acorrection function. The following will describe an embodiment of thepresent invention with reference to figures.

The optical pickup of the present embodiment can be represented by thesame block diagram as some of the foregoing optical pickups, i.e., FIG.6. The function of each block and member is not repeated here.

In the pickup 20, a beam expander 35 acts as correcting means correctingspherical aberration caused by the irregular thickness of a transparentsubstrate of an optical disc 40.

A position-on-focusing-trajectory signal FP, which will be subtractedfrom a focus error signal FE to obtain a focus error signal FE′, is asignal based on which a current focusing tracking actuator 27 is driven.Subtracting the position-on-focusing-trajectory signal FP from the focuserror signal FE yields a signal based on which the actuator is drivenbecause the focus error signal changes from the immediately precedingcondition to 0. When this signal is 0, this is interpreted as no focuserror having occurred; the focusing tracking actuator 27 is not driven,remaining in the same condition.

An objective lens 26 is moved by an amount in accordance with the focuserror signal FE, using the position on a focusing trajectory which isindicated by the position-on-focusing-trajectory signal FP in focusservo as a reference.

Results of experiments conducted by the inventors are shown in FIGS.14(a), 14(b) to FIGS. 16(a), 16(b). Figures 14(a), 14(b) show a statewhere the pickup causes no spherical aberration or focus offset. A RFsignal is recorded (test written) at the optimal write power. Thehorizontal axes indicate the magnitude of spherical aberration of therecorded signal. The vertical axes indicate its focus offset. Asreference signals, FIGS. 14(a), 14(b) show measurements of the jitterand maximum amplitude of a RF signal respectively in 2-dimensional maps.

Similarly to FIG. 18, the optical disc 4 here contains a 0.1 mm thicktransparent substrate and made of polycarbonate. The disc 4 has a trackpitch of 0.32 μm and a disc groove depth of 21 nm. The laser wavelengthof a measurement pickup is 405 nm, and the NA of the objective lens 27is 0.85. In the 2-dimensional maps, 6 points are plotted for themagnitude of spherical aberration ranging from −80 mλ to +80 mλ, and 11points are plotted for the focus offset ranging from −0.22 to +0.22 μm,which makes a total of 66 data points.

It is understood from FIGS. 14(a), 14(b) that the jitter is smallest atthe origin, and the maximum amplitude is greatest at the origin. Bothcharacteristics are represented by concentric circles around the origin.In the presence of spherical aberration and focus offset, a beam doesnot come in focus on the optical disc 40. As resolution thusdeteriorates, the beam spills into adjacent tracks and the preceding andsucceeding data records and dose not produce a well-focused image on thelight receiving elements A to D. This is the cause of the jitter.

These results make a good contrast to FIGS. 15(a), 15(b) where thepickup 20 causes a spherical aberration equivalent to a total of the CGthickness and +7 μm and a focus offset of 0.1 μm. A RF signal isrecorded at the optimal write power. The horizontal axes indicate themagnitude of spherical aberration of the recorded signal. The verticalaxes indicate its focus offset. As reference signals, FIGS. 15(a), 15(b)show measurements of the jitter and maximum amplitude of a RF signalrespectively in 2-dimensional maps. CG thickness refers to the thicknessof the transparent substrate (cover glass) on the recording surface ofthe disc. “The CG thickness and +7 μm” refers to a total of the designedcover glass thickness and +7 μm. When the design cover glass has athickness of 0.1 mm, the sum is 0.107 mm.

Similarly to FIGS. 14(a), 14(b), FIG. 15(a) and FIG. 15(b) show thesmallest jitter at the origin and the greatest maximum amplitude at theorigin. Both characteristics are represented by concentric circlesaround the origin. Therefore, the jitter and the maximum amplitude canbe converged at the origin even when neither the spherical aberrationnor the focus offset is optimal. In other words, the optimal values ofthe spherical aberration and the focus offset can be detected in singlecorrection steps, whichever value the magnitude of spherical aberrationassumes in focus offset correction and whichever value the focus offsetassumes in spherical aberration correction.

FIGS. 16(a), 16(b) are a case where the pickup causes a sphericalaberration equivalent to a total of the CG thickness and +7 μm and afocus offset of 0.1 μm. A RF signal is recorded at a write power 20%greater than the optimal value. The horizontal axes indicate themagnitude of spherical aberration of the recorded signal. The verticalaxes indicate its focus offset. As reference signals, FIGS. 16(a), 16(b)show measurements of the jitter and maximum amplitude of a RF signalrespectively in 2-dimensional maps.

It is understood from FIGS. 16(a), 16(b) that the relationship betweenthe spherical aberration and the focus offset is not affected byvariations in the write power in test writing.

Summing up the discussion, FIGS. 14(a), 14(b) show that when a RF signalrecorded under the optimal conditions is used as the reference signal,the jitter is a minimum and the RF signal amplitude is a maximum onlywhen there is zero spherical aberration and zero focus offset. FIGS.15(a), 15(b) show that when a RF signal recorded at some remainingspherical aberration and focus offset is used as the reference signal,the jitter is a minimum and the RF signal amplitude is a maximum onlywhen there is zero spherical aberration and zero focus offset. FIGS.16(a), 16(b) show that when a RF signal recorded at some remainingspherical aberration and focus offset and non-optimal write power isused as the reference signal, the jitter is a minimum and the RF signalamplitude is a maximum only when there is zero spherical aberration andzero focus offset.

Therefore, a RF signal which is recorded when the write power is yet tobe adjusted can be used as the reference signal in adjusting thespherical aberration and the focus offset. That is, the sphericalaberration and the focus offset can be independently adjusted regardlessof recording conditions. Now, these results will be utilized in thefollowing to devise procedures of adjusting the spherical aberration andthe focus offset.

FIG. 17 is a flow chart illustrating procedures of correcting thespherical aberration. The control circuit 50 executes control operationon the pickup 20 in accordance with a main routine (not shown) toimplement various read/write actions. If the optical disc 40 is insertedinto the read/write device while the control circuit 50 is executing themain routine, the control circuit 50 proceeds to execute a sphericalaberration correction subroutine illustrated in FIG. 17 before startingactual read/write actions.

First of all, in step S51, the control circuit 50 moves the pickup 20close to the center of the disc. The circuit 50 feeds a “logic 1” focusservo switching signal FS to the servo loop switch 5 to turn on thefocus servo and a “logic 1” tracking servo switching signal TS to theservo loop switch 6 to turn on the tracking servo.

In step S52, the circuit 50 reproduces address information and moves thepickup to the track where disc information is recorded as indicated bythe reproduced address information, so as to retrieve the write power,write pulse occur timings, and other information related to the writingof the disc. The address and disc information is frequency-modulated andrecorded in the wobbles of tracks when the disc is manufactured. It isreproduced, similarly to the tracking error signal TE′, from adifference signal, (RA+RD)−(RB+RC).

In step S53, the circuit 50 moves the pickup to a test write area totest-write data under the retrieved recording conditions. In next stepS54, the circuit 50 proceeds to a spherical aberration correctionroutine where the distance between the lens 32, 33 in the beam expander35 is corrected to maximize the signal amplitude of the test-written RFsignal. In step S55, the circuit 50 proceeds to a focus offsetcorrection routine where the focus offset is adjusted to maximize thesignal amplitude of the test-written RF signal. After step S55, thecontrol circuit 50 proceeds to step S56 to execute a write powercorrection subroutine where an optimal write power is determined toconclude preparation for data read/write.

Referring to FIGS. 16(a), 16(b), this correction routine is described.If in the test write in step S53, a RF signal is recorded at a focusoffset of 0.1 μm, a spherical aberration equivalent to the CG thickness+7 μm, and a write power 20% greater than the optimal value, step S54corrects only the spherical aberration to 0 and step S55 corrects thefocus offset to 0. That is, steps S54 and S55 converge the sphericalaberration and the focus offset to the center of the concentric circlesin FIGS. 16(a), 16(b).

The spherical-aberration correction routine in step S54 and the focusoffset correction routine in step S55 may be interchangeable. Eitherroutine can be implemented first before the other, still capable of theconverging to the center of concentric circles in FIG. 16.

An example of specific procedures of the spherical aberration correctionroutine in step S54 is already given with reference to FIG. 8. RF signallevel changes when there is spherical aberration remaining with theoptical pickup in FIG. 6 and the lens distance in the beam expander 35is varied to change the spherical aberration are, as mentioned earlier,shown in FIG. 7. In FIG. 7, the P-V value refers to a maximum minus aminimum of the aberration and either positive or negative as indicatedby a plus or a minus sign.

The FIG. 8 operation utilizes the RF signal amplitude in variousprocessing. In place of the RF signal amplitude, the jitter or errorrate may be used.

In addition, the position of the spherical aberration correction meanswhere the optimal spherical aberration is produced may be stored inmemory means (e.g., the position of the beam expander 35 may be storedin a memory) for later retrieval based on which a next sphericalaberration and focus offset correction steps are implemented. Thisapproach has advantages: for instance, the optimal value detection stepcan be quickly completed when compared with the use of a groundlessvalue as the magnitude of spherical aberration.

Hence, the optical pickup quickly and accurately corrects sphericalaberration and focus offset on the writeable optical disc 40. Theoptical pickup requires writing only once and at a single write power,quickly implementing corrections, whereas Tokukaisho 64-27030 must writea set of sectors at multiple write powers and read all those sectors toobtain an optimal magnitude of correction.

The description so far has assumed the use the amplitude of the RFsignal of which the quality must be ensured by the optical pickup as thereference signal in the correcting. The use leads to accuratecorrection, requires no complex signal processing, and can beimplemented on a simple circuit. However, as mentioned earlier, RFsignal jitter, error rate, and other alternatives are possible.

If jitter, which is highly related to RF signal quality, is used as thereference signal, the spherical aberration and the focus offset arecorrected to produce a minimum jitter. This arrangement improvescorrection accuracy over amplitude detection and still requiresrelatively simple signal processing, albeit not as simple as inamplitude detection.

Alternatively, if error rate, which is highly related to RF signalquality, is used as the reference signal, the spherical aberration andthe focus offset are corrected to produce a minimum error rate. Thisarrangement provides the highest level of accuracy and good sensitivityin correction, although it adds to the circuit size and is susceptibleto noise. The arrangement is especially effective in fine-tuning thefocus offset.

Alternatively, if the RF signal amplitude, the jitter, and the errorrate may be used together as the reference signals. For example, thespherical aberration is corrected based on the amplitude of the RFsignal, and the focus offset is corrected based on the jitter.

The structure of the optical pickup of the present embodiment is notlimited to the one shown in FIG. 6. The pickup may be arranged as inFIG. 1. In place of the beam expander 35, the FIG. 1 optical pickupincludes the liquid crystal panel 25 between the quarter-wave plate 24and the objective lens 26. The focusing tracking actuator 27 moves theseobjective lens 26 and liquid crystal panel 25 together. This kind ofoptical pickup was already detailed above.

In the spherical aberration correction utilizing the liquid crystalpanel 25, a desired magnitude of spherical aberration, which can cancelthe spherical aberration caused by the irregular thickness of thetransparent substrate, can be produced immediately with no mechanicalmotion as described earlier. External disturbances do not affect thepickup. The magnitude of spherical aberration for correction can beaccurately managed. However, when compared with the beam expander 35, ahigh level of accuracy is required in assembly and calibration. Also, avoltage needs be applied to the liquid crystal throughout the aberrationcorrection process, adding to power consumption.

The present embodiment provides an optical pickup, for read/write of awriteable optical disc, which quickly corrects the spherical aberrationand focus offset caused by an irregularity thickness of the cover of theoptical disc.

To achieve this end, the present embodiment is arranged as follows.FIGS. 16(a), 16(b) show measurements the RF signal jitter and maximumamplitude of a test-written RF signal in 2-dimensional mapsrespectively. The RF signal is test-written in the presence of sphericalaberration and focus offset and at a non-optimal write power. Thehorizontal axes indicate the magnitude of spherical aberration. Thevertical axes indicate the focus offset. The jitter is smallest at theorigin, and the maximum amplitude value is greatest at the origin. Bothcharacteristics are represented by concentric circles around the origin.The optimal value of either one of the characteristics is detectablewithout being affected, when the other one is not optimal. Therefore,the spherical aberration correction and the offset adjustment can beindependently carried out in respective single test writings.

As discussed in the foregoing, a method of correcting a sphericalaberration focus offset of an optical pickup of the present inventioninvolves: the step of recording a signal on a storage medium at apredetermined write power; the step of reproducing recorded informationfrom reflection from a recording surface of the storage medium; the stepof producing a spherical aberration in the presence of a predeterminedfocus offset and changing the magnitude of the spherical aberration; theoptimal spherical aberration detection step of detecting a sphericalaberration occurrence condition when the spherical aberration is aminimum; the step of producing a focus offset under the minimumspherical aberration occurrence condition and changing the magnitude ofthe focus offset; and the optimal focus offset detection step ofdetecting a focus offset occurrence condition when the focus offset is aminimum, wherein the magnitude of the spherical aberration and themagnitude of the focus offset obtained in the optimal sphericalaberration detection step and the optimal focus offset detection stepare used to correct the spherical aberration and the focus offset.

Another method of correcting a spherical aberration focus offset of anoptical pickup of the present invention involves: the step of recordinga signal on a storage medium at a predetermined write power; the step ofreproducing recorded information from reflection from a recordingsurface of the storage medium; the step of producing a focus offset inthe presence of a predetermined spherical aberration and changing themagnitude of the focus offset; the optimal focus offset detection stepof detecting a focus offset occurrence condition when the focus offsetis a minimum; the step of producing a spherical aberration under theminimum focus offset occurrence condition and changing the magnitude ofthe spherical aberration; and the optimal spherical aberration detectionstep of detecting a spherical aberration occurrence condition when thespherical aberration is a minimum, wherein the magnitude of thespherical aberration and the magnitude of the focus offset obtained inthe optimal focus offset detection step and the optimal sphericalaberration detection step are used to correct the spherical aberrationand the focus offset.

In the present embodiment, the control circuit 50 acts as the recordingcondition detecting means, the test write means, and the correctingmeans; the RF signal generator circuit 3 acts as the correcting means;the laser producing element 21 acts as the test write means; the beamexpander 25 acts as the correcting means; the beam expander actuator 34acts as the correcting means; the liquid crystal panel 25 acts as thecorrecting means; and the liquid crystal driver 4 acts as the correctingmeans and the liquid crystal drive circuit.

As in the foregoing, an optical pickup of the present invention includescorrecting means producing a spherical aberration which cancels aspherical aberration in an optical system for correction when the pickupprojects a collected beam onto a recording surface of an optical storagemedium to retrieve recorded information by means of the intensity ofreflection from the recording surface. The pickup is arranged so thatthe correcting means is capable of producing at least two sphericalaberrations of different magnitudes by means of a collected beam spot onthe recording surface of the optical storage medium so that themagnitudes are ¼ or more of the wavelength λ in P-V values or 1/14 ormore of the wavelength λ in standard deviation. The pickup is arrangedalso so that the pickup includes control means which: causes thecorrecting means to produce the two second spherical aberrations ofdifferent magnitudes; calculates an optimal magnitude of aberrationcorrection through a numeric evaluation based on an evaluation value ofa reference signal obtained by receiving the reflection of intensitiesin the presence of the spherical aberrations of such magnitudes; andcontrols the correcting means to carry out correction using the optimalmagnitude of aberration correction.

INDUSTRIAL APPLICABILITY

According to an optical pickup and spherical aberration correctionmethod for optical pickups of the present invention, the correctingmeans produces spherical aberrations of which the magnitudes are ¼ ormore of the wavelength λ in P-V values or 1/14 or more of the wavelengthλ in standard deviation. A part where the changes in the evaluationvalue of the reference signal show high sensitivity to changes inmagnitude of the spherical aberration can be utilized as a referencesignal obtainable corresponding to these magnitudes of the sphericalaberrations.

Therefore, the optimal magnitude of aberration correction calculatedthrough a numeric evaluation based on the evaluation value of thereference signal obtainable corresponding to the magnitudes of thespherical aberrations is not affected by noise, external disturbance,and other unwanted factors, allows for the determination of a singlevalue, and enables quick and more accurate spherical aberrationcorrection with an optimal magnitude of aberration correction.

In the method of correcting a spherical aberration focus offset of anoptical pickup and the optical pickup of the present invention, incorrecting the spherical aberration and the focus offset of the opticalpickup, the inventors varied two parameters, the spherical aberrationand the focus offset, independently and paid attention to a finding thatthe optimal value of either one of the parameters could be obtainedwithout being affected, when the other parameter was not optimal.Accordingly, in read operation, the spherical aberration is first variedto detect an optimal magnitude of spherical aberration. Subsequently,using that optimal magnitude of spherical aberration, the focus offsetis varied to detect an optimal magnitude of focus offset. Alternatively,the focus offset is first varied to detect an optimal magnitude of focusoffset. Subsequently, using that optimal magnitude of sphericalaberration, the spherical aberration is varied to detect an optimalmagnitude of spherical aberration.

Thus, the spherical aberration and focus offset of a writeable opticaldisc can be corrected quickly and accurately.

1. An optical pickup projecting a collected beam onto a recordingsurface of an optical storage medium to retrieve recorded information bymeans of an intensity of reflection from the recording surface, saidpickup correcting a first spherical aberration in an optical system byproducing at correcting means a second spherical aberration whichcancels the first spherical aberration, said pickup being characterizedin that: the correcting means is capable of producing at least twosecond spherical aberrations of different magnitudes by means of acollected beam spot on the recording surface of the optical storagemedium so that the magnitudes are ¼ or more of a wavelength λ in P-Vvalues or 1/14 or more of a wavelength λ in standard deviation; and saidpickup comprises control means which: causes the correcting means toproduce the at least two second spherical aberrations of differentmagnitudes; calculates an optimal magnitude of aberration correction forthe first spherical aberration through a numeric evaluation based on anevaluation value of a reference signal obtained by receiving reflectionof intensities in the presence of the spherical aberrations of suchmagnitudes; and controls the correcting means to carry out correctionusing the optimal magnitude of aberration correction.
 2. The opticalpickup as set forth in claim 1, wherein in the numeric evaluation, thecontrol means calculates an approximation curve from the at least twosecond spherical aberrations of different magnitudes produced by thecorrecting means and the evaluation value for these second sphericalaberrations and designates a peak or bottom position of theapproximation curve as the optimal magnitude of aberration correction.3. The optical pickup as set forth in claim 2, wherein the approximationcurve is a multiple term approximation curve.
 4. The optical pickup asset forth in claim 1, wherein the control means: causes the correctingmeans to produce the two second spherical aberrations of differentmagnitudes so that the two second spherical aberrations are separated by½ or more of a wavelength λ in P-V values and that the second sphericalaberrations have substantially equal evaluation values; calculates amean value of the two magnitudes of the spherical aberrations as thenumeric evaluation; and uses the mean value obtained in the mean valuecalculation as the optimal magnitude of aberration correction.
 5. Theoptical pickup as set forth in claim 1, wherein the control means:causes the correcting means to produce a second spherical aberration ofa first magnitude and a second spherical aberration of a secondmagnitude which is separated by ½ or more of a wavelength λ in P-Vvalues from the second spherical aberration of the first magnitude sothat the second spherical aberration of the second magnitude can producea reference signal having an evaluation value substantially equal tothat of a reference signal obtained in the production of the secondspherical aberration of the first magnitude; calculates a mean value ofthe second spherical aberrations of the first and second magnitudes asthe numeric evaluation; and uses the mean value obtained in the meanvalue calculation as the optimal magnitude of aberration correction. 6.The optical pickup as set forth in claim 1, wherein the correcting meansincludes: a liquid crystal panel containing a circular band oftransparent electrode provided on a liquid crystal layer filled withbirefringent liquid crystal; and a liquid crystal drive circuit applyingto the transparent electrode voltages corresponding to the at least twosecond spherical aberrations of different magnitudes.
 7. The opticalpickup as set forth in claim 1, wherein the correcting means is a beamexpander including a pair of lenses and capable of producing the secondspherical aberrations by varying a distance between the lenses.
 8. Theoptical pickup as set forth in claim 1, wherein the correcting means ispositioned on an optical path along which the beam projected onto therecording surface of the optical storage medium and the reflection fromthe recording surface travel.
 9. The optical pickup as set forth inclaim 1, wherein: the control means: causes the correcting means toproduce a second spherical aberration of a first magnitude and a secondspherical aberration of a second magnitude so that the second sphericalaberration of the second magnitude can produce a reference signal havingan evaluation value substantially equal to that of a reference signalobtained in the production of the second spherical aberration of thefirst magnitude; calculates a mean value of the second sphericalaberrations of the first and second magnitudes as the numericevaluation; and uses the mean value obtained in the mean valuecalculation as the optimal magnitude of aberration correction; and thefirst and second magnitudes are smaller than a maximum signal amplitudeby 5% or more.
 10. The optical pickup as set forth in claim 1, wherein:prior to adjustment of a focus offset, the control means: causes thecorrecting means to produce a second spherical aberration of a firstmagnitude and a second spherical aberration of a second magnitude sothat the second spherical aberration of the second magnitude can producea reference signal having an evaluation value substantially equal tothat of a reference signal obtained in the production of the secondspherical aberration of the first magnitude; calculates a mean value ofthe second spherical aberrations of the first and second magnitudes asthe numeric evaluation; and uses the mean value obtained in the meanvalue calculation as the optimal magnitude of aberration correction; andthe first and second magnitudes are smaller than a maximum signalamplitude by 10% or more.
 11. The optical pickup as set forth in claim1, wherein the reference signal is an information signal read from therecording surface of the optical storage medium, and an evaluation valueof the reference signal is an amplitude level.
 12. The optical pickup asset forth in claim 1, wherein the reference signal is a tracking errorsignal, and an evaluation value of the reference signal is an amplitudelevel.
 13. The optical pickup as set forth in claim 1, wherein thereference signal is an information signal, and an evaluation value ofthe reference signal is jitter.
 14. The optical pickup as set forth inclaim 1, wherein the reference signal is an information signal, and anevaluation value of the reference signal is an error rate.
 15. A methodof correcting a spherical aberration of an optical pickup, said methodcorrecting a first spherical aberration in an optical system byproducing a second spherical aberration which cancels the firstspherical aberration when the pickup projects a collected beam onto arecording surface of an optical storage medium to retrieve recordedinformation by means of an intensity of reflection from the recordingsurface, said method being characterized in that it comprises the stepsof: producing at least two second spherical aberrations of differentmagnitudes by means of a collected beam spot on the recording surface ofthe optical storage medium so that the magnitudes are ¼ or more of awavelength λ in P-V values or 1/14 or more of a wavelength λ in standarddeviation; calculating an optimal magnitude of aberration correction forthe first spherical aberration through a numeric evaluation based on anevaluation value of a reference signal obtained by receiving reflectionof intensities in the presence of the spherical aberrations of suchmagnitudes; and correcting the first spherical aberration using theoptimal magnitude of aberration correction.
 16. A method of correcting aspherical aberration focus offset of an optical pickup, said methodcorrecting a spherical aberration and a focus offset in an opticalsystem when the pickup projects a collected beam onto a recordingsurface of an optical storage medium to retrieve recorded information bymeans of an intensity of reflection from the recording surface, saidmethod being characterized in that it comprises: the step of recording asignal on the storage medium at a predetermined write power; the step ofreproducing the recorded information from the reflection; the step ofproducing a first correction target in the presence of a predeterminedsecond correction target and changing the first correction target, wherethe first correction target is either one of the focus offset and thespherical aberration, and the second correction target is the other one;the optimal first correction target detection step of detecting anoccurrence condition of the first correction target when the firstcorrection target is a minimum; the step of producing the secondcorrection target under an occurrence condition of the minimum firstcorrection target and changing a magnitude of the second correctiontarget; and the optimal second correction target detection step ofdetecting an occurrence condition of the second correction target whenthe second correction target is a minimum, wherein the magnitude of thespherical aberration and the magnitude of the focus offset obtained inthe first correction target detection step and the optimal secondcorrection target detection step are used to correct the sphericalaberration and the focus offset.
 17. A method of correcting a sphericalaberration focus offset of an optical pickup, said method correcting aspherical aberration and a focus offset in an optical system when thepickup projects a collected beam onto a recording surface of an opticalstorage medium to retrieve recorded information by means of an intensityof reflection from the recording surface, said method beingcharacterized in that it comprises: the step of recording a signal onthe storage medium at a predetermined write power; the step ofreproducing the recorded information from the reflection; the step ofproducing a spherical aberration in the presence of a predeterminedfocus offset and changing a magnitude of the spherical aberration; theoptimal spherical aberration detection step of detecting a sphericalaberration occurrence condition when the spherical aberration is aminimum; the step of producing a focus offset under the minimumspherical aberration occurrence condition and changing a magnitude ofthe focus offset; and the optimal focus offset detection step ofdetecting a focus offset occurrence condition when the focus offset is aminimum, wherein the magnitude of the spherical aberration and themagnitude of the focus offset obtained in the optimal sphericalaberration detection step and the optimal focus offset detection stepare used to correct the spherical aberration and the focus offset.
 18. Amethod of correcting a spherical aberration focus offset of an opticalpickup, said method correcting a spherical aberration and a focus offsetin an optical system when the pickup projects a collected beam onto arecording surface of an optical storage medium to retrieve recordedinformation by means of an intensity of reflection from the recordingsurface, said method being characterized in that it comprises: the stepof recording a signal on the storage medium at a predetermined writepower; the step of reproducing the recorded information from thereflection; the step of producing a focus offset in the presence of apredetermined spherical aberration and changing a magnitude of the focusoffset; the optimal focus offset detection step of detecting a focusoffset occurrence condition when the focus offset is a minimum; the stepof producing a spherical aberration under the minimum focus offsetoccurrence condition and changing a magnitude of the sphericalaberration; and the optimal spherical aberration detection step ofdetecting a spherical aberration occurrence condition when the sphericalaberration is a minimum, wherein the magnitude of the sphericalaberration and the magnitude of the focus offset obtained in the optimalfocus offset detection step and the optimal spherical aberrationdetection step are used to correct the spherical aberration and thefocus offset.
 19. The method as set forth in claim 16, wherein aspherical aberration and/or a focus offset are produced which maximizean amplitude of the reproduced signal.
 20. The method as set forth inclaim 16, wherein a spherical aberration and/or a focus offset areproduced which minimize a jitter of the reproduced signal.
 21. Themethod as set forth in claim 16, wherein a spherical aberration and/or afocus offset are produced which minimize an error rate of the reproducedsignal.
 22. An optical pickup including a correction device producing aspherical aberration and a focus offset which cancel a sphericalaberration and a focus offset in an optical system for correction whensaid pickup projects a collected beam onto a recording surface of anoptical storage medium to retrieve recorded information by means of anintensity of reflection from the recording surface, said pickup beingcharacterized in that said correction device comprises: recordingcondition detecting means detecting a recording condition recorded inadvance on the optical storage medium; test write means test-writing apredetermined signal in a test write area of the optical storage mediumunder the recording condition detected by the recording conditiondetecting means; and correcting means executing: the process ofproducing a first correction target in the presence of a predeterminedsecond correction target and changing the first correction target usinga reproduction signal from the test write area, where the firstcorrection target is either one of the focus offset and the sphericalaberration, and the second correction target is the other one; theoptimal first correction target detection process of detecting anoccurrence condition of the first correction target when the firstcorrection target is a minimum; the process of producing the secondcorrection target under an occurrence condition of the minimum firstcorrection target and changing a magnitude of the second correctiontarget; the optimal second correction target detection process ofdetecting an occurrence condition of the second correction target whenthe second correction target is a minimum; and the process of using amagnitude of the spherical aberration and a magnitude of the focusoffset obtained in the first correction target detection process and theoptimal second correction target detection process to correct thespherical aberration and the focus offset.
 23. The optical pickup as setforth in claim 22, wherein the correcting means is a beam expanderincluding a pair of lenses and matches a distance between the lenses tothe magnitude of the spherical aberration obtained in the optimalspherical aberration detection process.
 24. The optical pickup as setforth in claim 22, wherein the correcting means includes: a liquidcrystal panel containing a circular band of transparent electrodeprovided on a liquid crystal layer filled with birefringent liquidcrystal; and a liquid crystal drive circuit applying to the transparentelectrode voltages corresponding to the magnitude of the sphericalaberration obtained in the optimal spherical aberration detectionprocess.